The present invention relates to columns for use in gas chromatography and methods of making and using such columns.
Gas chromatography is a standard technique for separating compounds in gas samples for composition analysis. Typically, a gas sample is injected into a fused silica capillary column, and constituents of the sample separate based on each constituent compound's affinity for a coating on the interior wall of the capillary. Some compounds move rapidly through the column exhibiting little interaction with the column's coating, while other compounds move slowly through the column because of strong interactions with the coating.
Gas chromatography columns are typically 1 to 30 meters long, and have an inner diameter of 50 to 520 microns. Creating a compact gas chromatography (GC) column is challenging because it is difficult to bend fused silica capillaries to a radius much smaller than 100 mm. In the late 1990's, techniques were developed to etch spiral channels in silicon and cover the channel with a lid to produce rectangular cross-section columns (Overton, U.S. Pat. No. 6,068,684). Heaters or coolers are attached to these columns to provide controlled temperature profiles that aid in sample separations (Manginell, et al, U.S. Pat. No. 6,666,907 and Robinson, et al, U.S. Pat. No. 6,706,091).
Techniques have been developed to produce circular cross section columns in nickel from stacked sheets with an array of holes (U.S. Pat. No. 7,273,517). In this process, nickel is deposited on a plastic mold to form a thin sheet with an array of holes. Multiple sheets are stacked together to form an array of columns. Through the same deposition process, sheets with an array of slots are formed in nickel, and the slotted-sheets are stacked on the sheets with holes in such a way to form a continuous, serpentine passage. The entire stack is diffusion bonded together to create a single-chip GC column. With GC columns formed in this process, a 1-meter long column can be packaged in a chip that is approximately 13-mm on each side 1-mm thick. Longer columns have been formed by adding layers of hole-patterned sheets to the stack.
A drawback to the nickel micro-GC column is that multiple sheets with an array of holes are used in forming the stack. The thickness of the sheet is dictated by the time allocated to the deposition process; typically, each layer will be only 200 microns thick, so 5 layers would be required to make a 1 meter column. Each of these sheets must be accurately aligned in the stack to insure a uniform column, and each of the layers must be lapped flat and parallel to insure that the seal formed in diffusion bonding is hermetic. Also, the cross-section is not perfectly circular in the slotted passages that link holes in the array. Ideally, the cross-section would be uniform throughout the column to achieve the best chromatography. In a 30×30 array of these holes, there are 900 such slotted inter-linking passages that will perturb the flow profile through the nickel micro-GC column. Reducing the number of flow perturbations is important to improve chromatography.
There is a need for improved columns for use in gas chromatography.